Field of the Invention
The present invention concerns a method and apparatus for acquiring medical imaging data and for correlating the acquired medical imaging data with measurements of physiological characteristics of a patient.
Description of the Prior Art
In the diagnosis of Coronary Artery Disease (CAD), molecular imaging techniques are used to measure blood flow in the myocardium. Radiopharmaceuticals that act as tracers for blood are introduced into the patient and imaged using Single Photon Emission Computed Tomography (SPECT) or Positron Emission Tomography (PET) scanners.
In images produced by PET or SPECT scans, the myocardium shows up as a region of high activity where blood is being carried through the coronary arteries and into the wall of the heart. Areas of reduced flow are possible indicators of coronary vessel stenosis, which is characteristic of CAD.
In many cases, acquisitions are repeated both under conditions of rest and of stress. Measurement of blood flow under each may enhance the information available to the clinician. A condition of stress may be induced by exercising the patient on, for example, an exercise bike, or may be induced pharmaceutically.
One established technique for detecting disease in this domain is to compare measurements of blood flow in the patient under examination with a database of “normal” cases representing similar measurements on healthy individuals. This technique assists the physician in assessing whether detected variations in measured blood flow are clinically significant. The compared measurements, however, typically represent relative blood flow, which renders such comparison less than ideal.
A dynamic scan protocol involves acquisition of a number of time-stamped SPECT or PET images of the heart at different timepoints over a period of time. From a collection of such time-stamped images, one can determine the rate at which the tracer has entered different regions of the myocardium. Using kinetic modeling techniques known in themselves, this information can be used to derive measurements of absolute blood flow at particular times, which allows a more direct and meaningful comparison between the patient data acquisition and databases of “normal” cases.
A problem with this methodology is that the derived absolute blood flow rate is calculated only from a rate of change of SPECT or PET activity in the myocardium. Real absolute blood flow rate is also dependent on the patient's heart rate and blood pressure as well as blood vessel diameter at the time of acquisition. Various techniques are used to normalize the derived blood flow measurements against variations in these parameters.
However, such techniques involve the measurement of the relevant parameters—in this case, heart rate, blood pressure and vessel diameter—prior to, and/or after, the patient SPECT or PET data acquisition. No account is taken of changes in the values of these parameters between the time of their measurement and the time of the patient SPECT or PET data acquisition, which may, especially in the case of a stress scan, be a significant time.
A patient's Heart Rate (HR) and Blood Pressure (BP) may vary significantly over the course of the patient SPECT or PET data acquisition, particularly in the case of a dynamic cardiac acquisition, which may take many minutes. This variation will bias the results to emphasize data corresponding to time periods of higher blood pressure and heart rate.